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For questions involving one or more categorical dimensions, or involving homotopy coherent categorical structures.

60 votes
Accepted

What is the mistake in the proof of the Homotopy hypothesis by Kapranov and Voevodsky?

Here is my guess. To compare spaces with their notion of strict $\infty$-groupoids (in which everything is strict except inverses) Kapranov and Voevodsky use an intermediate category of Kan diagrammat …
Yonatan Harpaz's user avatar
21 votes
Accepted

What's the stabilization of the $\infty$-category of $\infty$-categories?

In a project in progress with Matan Prasma and Joost Nuiten concerning the abstract cotangent complex formalism we compute the stabilization of the $\infty$-category $\infty\mathrm{Cat}_{/C}$ of $\inf …
Yonatan Harpaz's user avatar
17 votes

Kan extensions in the $2$-category of monoidal categories

I believe that this is a particular case of Lurie's "operadic left Kan extension". We may identify a monoidal $\infty$-category $\mathcal{C}$ with a coCartesian fibrations of $\infty$-operads $\mathca …
Yonatan Harpaz's user avatar
14 votes
1 answer
485 views

Why is every object cofibrant in an excellent model category?

In Appendix A.3 of the book higher topos theory appears the notion of an excellent model category (see Definition A.3.2.16). The main feature of this notion is that when $\mathbf{S}$ is an excellent m …
Yonatan Harpaz's user avatar
13 votes
1 answer
474 views

Is the operadic nerve functor an equivalence of ∞-categories?

It is now known that the $\infty$-category of $\infty$-operads as defined by Lurie is equivalent to the underlying $\infty$-category of the model category of simplicial operads, see http://arxiv.org/p …
Yonatan Harpaz's user avatar
11 votes
1 answer
1k views

The universal property of the unseparated derived category

In Appendix C of his book in progress Spectral Algebraic Geometry, Lurie defines the unseparated derived category $\check{{\cal D}}({\cal A})$ (see Definition C.5.8.2 loc.cit) associated to a Grothend …
Yonatan Harpaz's user avatar
11 votes
Accepted

How can I functorially dualise in a symmetric monoidal $(\infty,1)$-category with duals?

One way to construct the duality functor ${\cal C} \to {\cal C^{\rm op}}$ is through the notion of a pairing of $\infty$-categories (see HA, Definition 5.2.1.5). In particular, in this case we're talk …
Yonatan Harpaz's user avatar
9 votes
Accepted

The cofibration/fibration $\leftrightarrow$ epi/mono confusion

The (epi,mono) factorization system in Sets is part of a model structure on Sets whose weak equivalences are the epis, fibrations are monos and cofibrations are everything. This is a model for the hom …
Yonatan Harpaz's user avatar
8 votes
Accepted

Lemma 5.4.5.11 of HTT

I think there is a typo in Lemma 5.4.5.11: $K$ is supposed to be $\tau$-small and not $\kappa$-small. Note that if $\tau < \kappa$ and $K$ is $\kappa$-small but not $\tau$-small then the statement of …
Yonatan Harpaz's user avatar
7 votes
Accepted

What is a spectrum object in $\infty$-topoi?

Following up on the answer of Simon Henry, let us prove the following statement. For a pro-space $\hat{X} = \{X_i\}_{i \in I}$, we let $Spaces_{/\hat{X}}$ denote the $\infty$-topos defined as the (co …
Yonatan Harpaz's user avatar
7 votes
Accepted

Methods for defining/calculating homotopy limits of quasicategories

When working with quasi-categories, it is often more convenient (and more compatible with existing machinery) not to work with actual strict diagrams of quasi-categories but rather with coCartesian fi …
Yonatan Harpaz's user avatar
6 votes
Accepted

strict 2-groups VS crossed modules

Let us prove the analogous claim for weak 2-groups (this implies, in particular, the strict case). Let $\mathcal{C}$ be a monoidal category with unit $\mathbb{I} \in \mathcal{C}$. Claim: Suppose the …
Yonatan Harpaz's user avatar
6 votes
Accepted

Spelling out explicitly the data of a two step filtration in terms of pieces and gluing data

Technically speaking the answer to your question is no, in the sense that the data of $(\alpha,\beta,\gamma,\delta)$ alone does not determine the filtered object $V_0 \subseteq V_1 \subseteq V_2$. How …
Yonatan Harpaz's user avatar
6 votes

Theorem 2.1.2.2 Higher Topos Theory

I think what Lurie might have meant when he wrote "It is easy to see that $St_{\phi}$ preserves cofibrations" in the proof of Theorem 2.2.1.2, is that it is easy to see it if you take into account the …
Yonatan Harpaz's user avatar
6 votes
Accepted

The naive approach to deriving profunctors - What's wrong with it?

The problem with this definition is that the formula $\mathrm{colim}_{X \to Z \in \mathcal{W}} F(Z)$ does not, in general, depend functorially on $X$. For it to depend functorially on $X$ you need a w …
Yonatan Harpaz's user avatar

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